1. Field of the Invention
This invention relates to a process for preparing heteropoly-acids and more particularly to a process for preparing a free heteropolyphosphoric acid containing molybdenum, vanadium and optionally tungsten as peripheral coordinate atoms. In one aspect this invention relates to a process for preparing a heteropoly-acid catalyst comprising such a heteropolyphosphoric acid supported on a carrier.
2. Description of the Prior Art
Heteropolyphosphoric acids are generally water-soluble substances having a structure in which oxy anions of peripheral coordinate atoms such as molybdenum, vanadium, tungsten, etc. are condensed about a phosphorus cation as the central element with a certain regularity. Those heteropolyphosphoric acids containing molybdenum and vanadium as peripheral coordinate atoms, that is, molybdovanadophosphoric acids, are useful as catalysts for oxidation, etc., but their synthesis involves some difficulties. Heretofore the following methods have been known for the synthesis of molybdovanadophosphoric acids.
(1) An aqueous slurry is prepared from an alkali vanadate, phosphoric acid and molybdic acid and heated. Thereafter hydrochloric acid is added and the mixture is extracted with ether; Zh. Obshch. Khim., 24, 966 (1954).
(2) Sulfuric acid is added to an aqueous solution of alkali vanadate, alkali phosphate and alkali molybdate and the mixture is then extracted with ether; Inorg. Chem., 7, 437 (1968).
While the above methods (1) and (2) are effective for reducing the time of reaction, it is necessary to use an alkali vanadate as the vanadium source or otherwise vanadium oxide must be homogeneously dissolved in an aqueous alkali solution prior to the condensation reaction. In addition, the vanadium source must be used in stoichiometrically large excess. These prior art methods are also disadvantageous in commercial operations, since the removal of the alkali residue requires complicated procedures involving strong acidification of the slurry or solution with excess mineral acid and subsequent extraction thereof with an organic solvent such as ether, as well as an additional step of treating the waste acid. In order to avoid these disadvantages, it has been proposed to calcine an intimate mixture consisting of a molybdenum compound, a vanadium compound and a phosphorus compound at an elevated temperature in an oxidizing atmosphere and subsequently extracting the mixture with water (Japanese Patent Laying-Open Publication No. 133,298/74). This method, however, has failed to completely solve the problem of the slow extraction rates involved therein.
Another method has been proposed which is directed to the preparation of alkali metal salts of molybdovanadophosphoric acids by heating an aqueous slurry containing an alkali vanadate, an alkali phosphate and molybdenum oxide, adding an alkali hydroxide to solubilize the solids in the slurry and then adding a mineral acid (British Pat. No. 1,376,432). This method, however, can not yield a free molybdovanadophosphoric acid which is effective as an oxidation catalyst.